This invention relates to phase comparison radio navigation systems of the kind in which spaced transmitting stations radiate phase-locked signals in a time-shared sequence and in which, at a receiver, the signals from the spaced stations are compared in phase.
Time-shared sequential transmissions from spaced transmitting stations are employed in many phase comparison radio navigation systems. They enable signals of the same frequency or frequencies to be radiated from each of a number of different stations to provide, at a receiver, signals from spaced stations which can be compared in phase to give positional information. In the receiver, it is necessary to identify the various signals and switch them to appropriate circuits. The same problem arises in secondary or slave stations where signals from a prime or master station have to be received to control the timing of the transmissions from the secondary or slave stations.
Heretofore it has been the usual practice periodically to modify the radiated signal from one of the stations in order to provide a timed identification. This modification might be constituted by a short duration break in the transmissions or by some other form of modulation such as a short duration change in frequency or a short duration change in phase. The timed identification signal can be received in a mobile receiver (and also at a secondary station) and is then used to initiate the necessary timing operations. The practice usually is to transmit an identification signal once in each cycle of the timing.
Particularly for a receiver at long ranges from the prime transmitting station, fading may occur and it is generally preferred nowadays to use a distinctive modulation and not merely a break in transmissions. However if such a modulation is to be received, it is necessary that the bandwidth of the receiver should be wider than would otherwise be necessary for the reception of an unmodulated signal. The wider the bandwidth, the greater is the amount of noise received and hence at long ranges, noise may cause difficulty in obtaining the required synchronising signal. Irrespective of bandwidth considerations, a break in transmissions is difficult to detect under conditions of fading and noise.